Microwave absorption by water in building materials

Abstract

Measurements are reported of the complex dielectric constants at wavelengths 10.0, 3.3 and 1.25 cm of specimens of concrete, mortar, hardened cement paste and brick (Hamill, Leicester Red, Burwell Gault and Fletton), loaded with water. Those in three of the bricks (H, L.R., B.G.) are interpreted in terms of dielectric mixture theory of three components, brick, air and rotationally free water absorbed in the cavities. Dissolved salt ionic conductivity is found to be unimportant; anisotropy at 10 cm wavelength is found in the Leicester Red brick. The dielectric properties of cement paste are believed to arise from water in the same state as that which produces the low frequency loss, rather than from rotationally free water. Absorption of free waves by walls is calculated from the data, and corresponds with experiment for concrete, but not for brick walls, on account of diffraction at mortar interstices.